An avalanche photodiode (APD) is a photodetector device, which multiplies carriers (electrons and holes) generated by light absorption through an avalanche mechanism and is used as an optical receiver with a low noise by taking out its output current. Recent APDs for a long wavelength band generally have a separated absorption and multiplication (SAM) structure, where a light absorbing layer and an avalanche multiplication layer are separated. In this SAM structure, in order to control electric field intensities of the light absorbing layer and the avalanche multiplication layer, an electric field control layer and a graded bandgap layer are provided between the two layers.
APDs are widely introduced to systems of 2.5 Gbit/s and 10 Gbit/s and are in a midst of development as elements for 40 Gbit/s system of the next generation.
In a technical field of such ultra high speed APD, APDs of “electron injection type”, which are advantageous from the view points of high speed operation, are attracting attention rather than those of “hole injection type” adopting InP, which is a structure hitherto been typically used as a structure for relatively slow speed operation, as the avalanche multiplication layer. Typical APDs of electron injection type reported so far are those having depleted InGaAs as the light absorbing layer and the InAlAs as the avalanche multiplication layer, respectively.
FIG. 1 is a band diagram of the APD of such electron injection type under operating conditions. In this diagram, reference numerals 41, 42, 43, 44, 45, 46, and 47 denote an n-type electrode layer, an avalanche multiplication layer (InAlAs), an electric field control layer, a graded bandgap layer, a low-doped light absorbing layer (InGaAs), a p-type electrode layer, and a p electrode, respectively. Note that the light absorbing layer 45 is depleted throughout its entire region.
A structure of the APDs of such “electron injection type” is advantageous in the high speed operation. However, on the other hand, since a bandgap of InAlAs used as the avalanche multiplication layer is larger than that of InP, which has been used as the avalanche multiplication layer in a “hole injection type”, the reduction in ionisation coefficient when a constant electric field intensity is applied, is inevitable and there is a problem that an operating voltage of the device is increased.
Apart from such a structure, a structure of the APD of “electron injection type” where the light absorbing layer is constituted of a p-type neutral layer (undepleted region) and a neighbouring thin low concentration layer (depleted region), and making the p-type neutral layer which is the undepleted region as the major light absorbing layer, is also reported (see Document 1).
FIG. 2 is a band diagram of such an APD of electron injection type under operating conditions. In this diagram, reference numerals 51, 52, 53, 54, 55, 56, 57, and 58 denote an n-type electrode layer, an avalanche multiplication layer, an electric field control layer, a bandgap inclined layer, a low concentration light absorbing layer (low concentration layer), a p-type light absorbing layer (p-type neutral layer), a p-type electrode layer, and a p electrode, respectively. Note that the p-type neutral layer, which is a non-depleted region, is an InGaAs layer.
A light absorbing layer of electron injection type APD of this structure is mostly occupied by a p-type light absorbing layer 56, which is the undepleted region. In other words, this structure is a “structure of making the light absorbing layer as p-type as much as possible.” Although the major advantage of the APD with a structure shown in this figure is the dark current reduction, it is also an effective structure for reduction in an operating voltage.
Determination of the light absorbing layer thickness is important in order to obtain a desired performance of the APD. If a carrier generation rate (quantum efficiency) is not high during a state where the avalanche multiplication is absent (a pin photodiode operation), a high S/N ratio cannot be ensured even if the avalanche multiplication is carried out. Thus, this is the reason why designing a thickness (WA) of light absorbing layer as thick as possible in a condition of a frequency response bandwidth needed to be ensured.
However, when an attempt is made to realize an operating speed of 10 Gbit/s or higher with the structure shown in FIG. 2 where the p-type neutral layer is the main light absorbing layer, a problem of reduction in a light absorption efficiency (quantum efficiency) arises due to a trade-off relationship between a carrier transit time and a quantum efficiency. This is caused by a fact that the carrier velocity in the p-type neutral InGaAs layer is usually smaller than that in the depleted InGaAs layer. In other words, this is because when the carrier transit time is designed so as to become equal to or lower than a certain value, an upper limit of a thickness of the p-type neutral InGaAs layer (p-type neutral layer) becomes thinner than that when using a depleted InGaAs layer.
Semiquantitative estimation for the frequency response bandwidth as a function of the light absorbing layer thickness will be described below.
The APD can be considered as a structure where a relatively thin avalanche multiplication layer is connected to a pin-type photodiode. Its bandwidth gradually decreases from an intrinsic bandwidth (intrinsic 3 dB bandwidth) in a state operating as the pin-photodiode and in then gradually approaches along a line of constant gain-bandwidth product as the avalanche multiplication factor increases. It is important to maintain the intrinsic 3 dB bandwidth high enough during the pin-photodiode operation together with the gain-bandwidth product high in order to obtain appropriately high gain. The intrinsic 3 dB bandwidth during the pin-photodiode operation is determined by the carrier transit time in the light absorbing layer and the multiplication layer. However, since the multiplication layer is far thinner than the light absorbing layer in a normal APD structure, the carrier transit time in the light absorbing layer is a dominant factor giving the intrinsic 3 dB bandwidth.
A multiplication layer structure can be designed almost independently from the light absorbing layer and it can be considered that the carrier transit time in the multiplication layer is commonly added. Thus, a bandwidth when taking only by the light absorbing layer into account is considered here. A saturation velocity (vh=5×106 cm/s) of holes is far smaller than that of electrons. Therefore, when it is approximated that the carrier transit time tD in a structure (light absorbing layer thickness WAD) where all the light absorbing layers are depleted, is determined by vh, according to a charge control model, Formula (1) can be obtained.tD=WAD/3vh  (1)
Moreover, 3 dB bandwidth (f3dB) is given by Formula (2).f3dB,D=1/[2πtD]=[1/WAD(μm)]×24 GHz  (2)
For example, when considering a margin in device design, WAD needs to be approximately 1.2 μm since f3dB,D=20 GHz is a measure for the 3 dB bandwidth of APDs receiving 10 Gbit/s signals. In order to maintain the hole saturation velocity throughout the entire region of this WAD, the electric field intensity needs to be 50 kV/cm or higher, in other words, a voltage needs to be at least 6 V or higher. Accordingly, since the electric field intensity of the light absorbing layer at the bias voltage for the avalanche multiplication is normally designed to be approximately 100 kV/cm, a voltage drop over the light absorbing layer part becomes 12 V, which is considerably large.
On the other hand, when the light absorbing part is only of a p-type neutral layer (with a constant concentration and a thickness of WAN), the carrier transit time τN is determined by a diffusion time of electrons. Since holes generated in the p-type light absorbing layer are majority carriers, they respond in order to maintain charge neutrality not as hole motion but as a hole current. Hence, a hole transport does not participate directly in a response speed. Assuming a diffusion coefficient of electrons to be De, the carrier transit time (tN) is derived by Formula (3).tN=WAN2/3De  (3)
The 3 dB bandwidth (f3dB) is approximated by Formula (4).f3dB,N=1/[2πtN]  (4)
When InGaAs with a doping concentration of 3×1017 cm3 is used for the light absorbing layer, an electron mobility is 6,000 cm2/Vs and a diffusion coefficient is approximately 150 cm2/s. Then the following formula is established.f3dB,N=[1/WAN2(μm2)]×7.2 GHz  (5)
In a similar way, when considering f3dB,N=20 GHz as a measure, WAN needs to be approximately 0.6 μm or lower. When the p-type neutral light absorbing layer is used, it is advantageous for reducing an APD operating voltage since the voltage for a carrier transit is not required. On the other hand, since the light absorbing layer thickness is relatively thin being 0.6 μm, which is about a half of that of the depleted light absorbing layer, a quantum efficiency of 1.5 μm band remains 50% or less and it becomes difficult to realize an APD with a high sensitivity.
As described so far, when an attempt is made to realize a reduction in the operating voltage, which is desired in APDs by using p-type a neutral light absorbing layer, a problem of a reduction in a quantum efficiency of devices operating in a high speed of 10 Gbit/s or higher arises.
Document 1: Japanese Patent No. 3141847